I think there are some other threads about this somewhere here. I'll try to find them. In the meantime....
I'll do this for
you - Lo haré para
tí. (
you are the beneficiary)
I'll do this for
you - Lo haré por
tí. (
I'm doing it in your place)
Think of para as passing the benefit fo the action to the other person. Think of it as directional. Indeed you can even use it like this:
Voy para la ciudad. - I'm going into town.
Think of por as causitive (for the cause of the other person, you're doing something.) Por also means 'by', 'because of' etc.
escrito por ... written by
No vino por (a causa de) la lluvia - He didn't come because it was raining.
Hope that helps a bit.
